Ink jet printing is a non-impact printing process in which droplets of ink are deposited on a substrate such as paper or transparency film in response to an electronic signal. Low cost and high quality of the output, combined with relatively noise free operation, have made ink jet printers a popular option to other types of printers used with computers. In thermal ink jet printing, a resistor element in a chamber is provided with an egress for the ink to enter from a plenum. The plenum is connected to an ink reservoir. A plurality of such resistor elements are arranged in a particular pattern in a print head. Each resistor element is associated with a nozzle in a nozzle plate, through which ink is expelled onto the print medium. The entire assembly of print head and reservoir comprise a thermal ink jet pen.
In operation, each resistor element is connected to a microprocessor, whose signals cause one or more elements to heat up rapidly. The heat creates a vapor bubble in the chamber, which expels ink through the nozzle onto the print medium. Firing of a plurality of such resistor elements in a particular order forms alphanumeric characters, area fills, or other indicia on the print medium.
Both dyes and pigments have been used as ink colorants for ink jet printers. However, dye-based inks generally suffer from deficiencies in water fastness, and lightfastness. Pigments are a preferred alternative to dyes, provided the pigment dispersions can be made stable to flocculation and settling.
Water based pigment dispersions are well known in the art, and have been used commercially for applying films such as paints onto various substrates. They are generally stabilized by ionic or non-ionic dispersants. While the pigments are inherently more lightfast than the dyes, the choice of the dispersant has a direct effect on ink properties such as waterfastness and smear resistance.
Three attributes of ink jet inks of prime importance are 1) print quality they produce, 2) drying rate when placed on the substrate, and 3) reliability. Print quality is mostly determined by factors such as optical density of the print, edge acuity, dot shape, and spray, if any. To a lesser degree, print quality is also determined by the printer mechanics such as resolution, drop volume, drop velocity, and the software controlling the print modes. But, it is the ink/paper interaction that has the dominant influence on print quality. Paper quality varies depending on the manufacturing process. An ink therefore would interact differently with different commercial papers. Inks that produce high print quality independent of the print media are hence highly desirable.
Drying rate, among other factors, determines the printer throughput. Ink jet printer throughput is relatively slow compared to printers using competing technologies. This is mainly due to "slow" drying rate of the ink jet inks. Inks usually dry by two mechanisms: evaporation and penetration. Evaporation is determined by the vehicle vapor pressure, whereas penetration is determined by the interfacial energy between the ink and the paper and the porosity of the paper.
Heating devices may be used to enhance drying. However, this also accelerates solvent evaporation from the nozzles and causes nozzle plugging. Heaters also add to the printer cost. The use of penetrating cosolvents and surfactants as means of improving dry time is well known in the art. However, many known penetrants, such as butyl carbitol, tend to destabilize pigment dispersions; surfactants also destabilize and/or increase feathering, resulting in degraded print quality.
The ink related reliability issues include 1) decap or crusting time which is defined as the time period over which a print head can stay dormant and exposed to the atmosphere without failing to print; 2) drop volume consistency over pen life; 3) compatibility with the print head components; 4) long term storage stability; and 5) robustness towards variable print parameters such as the firing frequency and pulse width.
The three major ink attributes discussed above conflict with each other. For example, methods known to increase drying rates such as the use of volatile cosolvents adversely affect either print quality by causing increased feathering or reliability by adversely affecting the crusting performance. U.S. Pat. No. 4,986,850 teaches that improved print optical density and edge acuity results from the use of up to 4.5% by weight of an alkylene oxide adduct of a polyhydric alcohol in the ink, which increases the viscosity of the ink due to solvent evaporation when printed on the substrate. This approach, however, also results in longer drying times and an increase in ink viscosity in the nozzles during dormant periods, which causes print reliability problems.
Accordingly, a need exists for inks that will produce high optical density, high print quality and permanent prints over a wide range of papers. A need also exists for inks that dry fast, print reliably and are stable for long periods of storage. This invention satisfies these requirements by using a stable pigment dispersion and a cosolvent system which makes the ink penetrate effectively for achieving the required balance between dry time and spreading.